1. The Field of the Invention
The present invention relates generally to the field of fiber optic communication. More particularly, the present invention relates to an optical fiber stub having a fiber within a ferrule, wherein the ferrule has an index of refraction higher than that of at least a portion of the fiber core.
2. Related Technology
Communications networks continue to develop and expand due to declining costs, improved performance of computer and networking equipment, and growth of the Internet. One type of communication network of increased importance is fiber optic communications networks. Fiber optic communications networks include communications systems and components in which optical fibers are used to carry signals form point to point. Optical fibers for carrying such signals include filaments or fibers made of dielectric materials that guide light.
A small core optical fiber, for example about 8-9 microns, typically carries a single-mode and is therefore termed single-mode fiber. Such single-mode fibers are well suited for long transmission distances because all of the light travels within the fiber along a well defined trajectory. A larger core diameter fiber, for example 62.5 microns, can propagate more than one mode of light and is therefore termed a multi-mode fiber. Multi-mode fiber is well suited to shorter transmission distances, for example within buildings, while single-mode fiber is well suited to longer transmission distances such as long-distance telephony and cable television systems.
Single-mode fibers have several advantages over multi-mode fibers. For example, single-mode fibers allow for a higher bandwidth-distance capacity to transmit information because they retain the fidelity of each light pulse over longer distances. Single-mode fibers also do not exhibit modal dispersion caused by differences between tolerances needed to make fiber optic connectors and the corresponding receptacles on fiber optic transceivers. However, single-mode fibers have been expensive, which has made them impractical for cost sensitive short distance data links.
For these reasons, multi-mode fibers became, and to a large extent remain, the practical standard for fiber optic cabling within short distance applications, such as local area networks within office buildings and the like. This has led to a large infrastructure of legacy multi-mode fiber and a corresponding desire to use this infrastructure for newer higher speed links. As a result, as well as for other reasons, many fiber optics transceivers are required to work with both single-mode and multi-mode fibers.
To communicate over a network using fiber optic technology, fiber optic components such as fiber optic transceivers are used to send and receive optical data signals. Generally, a fiber optic transceiver can include one or more optical subassemblies (“OSA”) such as a transmitter optical subassembly (“TOSA”) for sending optical signals, and a receiver optical subassembly (“ROSA”) for receiving optical data signals. Both the ROSA and the TOSA include specific optical components for performing such functions. In particular, a typical TOSA includes an optical transmitter, such as a laser, for sending an optical signal. Many different types of lasers are known to those skilled in the art. For example, there are edge emitting lasers, such as double heterostructure, quantum well, strained layer, distributed feedback, and distributed Bragg reflector lasers. Another type of laser referred to as a vertical cavity surface emitting laser (“VCSEL”) emits light in a single direction through an upper surface of the laser structure.
As described above, many fiber optic transceivers are required to work with both single-mode and multi-mode fibers. Because the core of single-mode fibers is relatively small, typically around 9 microns, light coupling to a single-mode fiber can be challenging. Multi-mode fibers, on the other hand, have relatively large diameters, typically 50 or 62.5 microns. Thus, because multi-mode fibers have a diameter that is on the order of six times greater than that of single-mode fibers there is about a 30 to 35 time larger core area for multi-mode fibers to receive light than that for single-mode-fibers. As a result, multi-mode fibers tend to couple significantly more light than single-mode fibers from the same light source.
There are relatively tight industry standards for coupling optical power into optical fibers. Therefore, controlling optical power coupling for both single-mode and multi-mode fibers can present problems for transceiver design. Because of the small diameter of single-mode fibers, single-mode fibers project well focused light in a certain area of the multi-mode fiber for optimal link performance. One example of a requirement for projection of light between optical fibers is the IEEE 802.3aq standard, which specifies that the light is to be focused to the center 10 microns of the multi-mode fibers. Too much light outside of the 10 micron diameter, but still within the 50 micron or 62.5 micron diameter of the multi-mode fiber, seriously degrades the 300 meter multi-mode legacy fiber link performance, due to fiber manufacturing imperfections.
One attempt at resolving this problem involves the use of a short single-mode fiber stub at the end of the transceiver light source. Light is focused onto the core of a single-mode fiber within the fiber stub. The fiber stub mates with an external fiber, and ideally, if the light is confined well within the core, all of the light is coupled into the center of a core of the external fiber. This should be true whether or not the external fiber is a single-mode fiber, or multi-mode fiber.
Unfortunately, this approach does not work well because the length of the fiber stub is limited to a few millimeters in conventional transceivers. Depending on considerations such as the laser beam profile, lens design, and alignment tolerances, centering the focal spot of the light entering the core of the fiber stub can be complicated. Where misalignment occurs, a significant portion of the light is coupled into the cladding of the fiber stub. This light can also be referred to as cladding modes, and the cladding modes are reflected off the walls of the ferrule in which the stub is positioned. If the fiber is relatively long, the cladding modes tend to die out by the time the light reaches the opposite end of the fiber. In this instance, only light in the fiber core is propagated out to the external optical fiber. In the case of a short fiber stub, however, much of the light in the cladding survives, and a portion of that light may be coupled into the endface of the external multi-mode fiber core by reflecting off cladding walls of the fiber stub and being received within the multi-mode core of the external optical fiber.
The optical power transmitted through the fiber stub cladding and coupled into the core of the external multi-mode fiber represents the coupling difference between external single-mode and multi-mode fibers and can range from 1 dB up to 5-6/dB, depending on the laser focal spot beam quality. Thus, what would be advantageous is to reduce, or eliminate, the transfer of cladding modes of light between optical fibers.